Quite a number of processes directed to the epoxidization of olefins are known, although the greatest part of them are methods which either did not find a practical industrial application or are now of no interest because not possessing, to a sufficient extent, all of the requisites of an applicable and economical character and, lately, also of ecological acceptability, etc., necessary for rendering them entirely acceptable for industrial use.
Even at the present time, besides the direct oxidation of ethylene to ethylene oxide, the propylene oxide, and at any rate the epoxides in general, are obtained almost exclusively by the known chlorohydrin process.
Schematically, that process consists in reacting an olefin with chlorine water obtaining thereby the chlorohydrin which is treated successively with alkalis (lime) thereby obtaining the corresponding epoxide. However, the process is subject to ever-increasing difficulties from the industrial point of view as well as on the economical plane, and with respect to environmental compatibility. In fact, the chlorohydrin process leads to the contemporaneous, more or less abundant and difficult to control, production of both mineral as well as organic chlorinated by-products which, while not useful, present major problems in the qualitative and quantitative disposal thereof.
To this must be added the growing cost of the chlorine, strictly bound to energy consumption, and other factors.
Therefore, quite recently interest was aroused in the epoxidization of an olefin by a process conducted in anhydrous organic phase with an organic hydroperoxide and in the presence of catalysts based on molybdenum, tungsten or vanadium, of which there have been some actual industrial applications.
However, that process has the disadvantage that production of the epoxide is accompanied by the production of the alcohol corresponding to the hydroperoxide, in quantities equivalent to or even greater than the epoxide. Exploitation of the alcohol as such, or recycle thereof, represents serious economical burdens which substantially affect commercial use of this process.
Research has been directed, also, to more direct oxidation methods.
Thus, epoxidization processes by means of molecular oxygen with silver catalysts, etc., have been proposed. Those processes had only limited success in the case of ethylene. The techniques proved to be not extendible to other olefins of significant interest, e.g., propylene.
Hydrogen peroxide, due to its oxidizing action associated with the absence of environmental problems, problems of pollution, etc., has been suggested as suitable for a certain number of epoxidization processes.
According to those processes, since the activity of the hydrogen peroxide toward the olefins as such is rather limited or altogether absent, it is necessary to use activating agents, in general organic acids such as for instance formic acid, acetic acid, etc., in organic solvents, which acids, in the form of peracids, form "in situ" the reactive epoxidizing agent.
Those processes, also, do not seem to have achieved any real success, both because of the difficulty of obtaining peracids as well as because of the instability of the epoxides in an acid medium which makes necessary rather burdensome operating conditions.
Other processes have been described as applicable selectively to the preparation of epoxy-alcohols (glycydols) by an epoxidization of hydrosoluble olefins with hydrogen peroxide, in an aqueous solution containing primary or secondary alcohols, and in the presence of catalysts based on: Mo, V, W.
In this last-mentioned case, it is a question of a technique substantially directed towards glycydols only, compounds that are of a limited commercial interest. On the other hand, the opoxidization reaction of the olefins with hydrogen peroxide leads to the formation of water which, especially when metal catalysts are used in the form of peroxides inhibits, with its accumulations, the reaction itself, a drawback which it was attempted to obviate by the use of concentrated solutions of hydrogen peroxide, as well as by using potentiated catalytic systems.
Thus, there have been described olefins oxidizable by reaction with highly concentrated hydrogen peroxide in a homogeneous, essentially organic liquid phase, in the presence of catalytic systems soluble in the organic liquid and based on elements of the IV, V and VI B (Ti, V, Mo, W) groups, of the Periodic Classification of Elements associated with elements selected from Pb, Sn, As, Sb, Bi, Hg, and so on.
The results did not meet expectations on the practical level because of the slowness of the reaction and of the expensiveness of the catalytic system consisting, in general, of very sophisticated organometallic compounds, necessary for their solubility in the organic medium.
Moreover, the use of hydrogen peroxide of high concentration (&gt;70%) involves some risks from the point of view of safety, not easily economically surmountable.
Improvements have been described as achievable by the use, in the above-mentioned technique, of catalysts based on tungsten or molybdenum or of arsenic or boron with an excess of olefin, in general, in combination with continuous distillation of the interfering water.
Here, also, the use of concentrated solutions (&gt;70%) of hydrogen peroxide is practically required, with the corresponding handling problems connected with it as already mentioned with regard to the safety of the installations. Moreover, the continuous removal of the reaction water besides that introduced by the hydrogen peroxide itself, an operation that makes the high H.sub.2 O.sub.2 concentrations practically necessary from the start, proves particularly economically burdensome.
On the other hand, the oxidization of the olefins with hydrogen peroxide involves an intrinsic contradiction resulting from the operating conditions which require at best an aqueous medium, possibly acid, as far as the catalytic system and the hydrogen peroxide are concerned, while the oxidization reaction and the stability of the epoxide require preferably a neutral organic medium.
All of the prior processes discussed above, and which involve the use of hydrogen peroxide, substantially tend to make the reaction medium homogeneous in some way and also tend to avoid the inhibiting accumulation of H.sub.2 O, with rather uncertain results, at least from the point of view of the actual industrial feasibility of such processes.
It is known from the literature to conduct chemical reactions in general substantially based on the ionic exchange, according to the so-called "double phase" technique.
There has also been described the possibility of epoxidizing olefins with hydrogen peroxide, in a double phase, in the presence of mineral derivatives based on tungsten and molybdenum. We are not aware of any practical interest in the method because of the poor efficiency of the catalyst, also verified by us, and therefore the double-phase technique has not appeared to be at all convenient or commercially feasible.